The present invention relates generally to ATM (Asynchronous Transfer Mode) transmission, and more particularly to a method and a device for the provision of ATM cells at an interface arranged between a first and a second node of a transmission system.
As a rule, in ATM systems, the data are transmitted in cells having a length of 53 bytes, whereby 5 bytes are reserved for the header and 48 bytes for the payload. Each cell has address information comprising two address identifiers, namely the VPI (Virtual Path Identifier) and the VCI (Virtual Channel Identifier). Moreover, the cell header comprises a Payload Type Identifier (PTI), which comprises three bits. According to the OSI reference model, there is envisaged a layer, namely the so-called ATM Adaption Layer (AAL), which controls the transmission of large data packets. This involves the use of two different encoding rules, namely AAL2 and AAL5.
FIG. 1 shows, as an example, the known configuration of ATM cells for AAL5: Each ATM cell ATM1, ATM2, ATM3 exhibits a header H1, H2, H3 and a payload D1, D2, D3. The last cell of a transmission further comprises a trailer T. In the case of AAL5, said last cell is characterised by the Payload Type Identifier “001” or “011”. For all other cells, the Payload Type Identifier has the value of “000” or “010” for AAL5. The ATM cells ATM1, ATM2, ATM3, which belong to the same ATM channel are grouped together, see FIG. 1. Accordingly, for AAL5, all information having the same VPI/VCI address is designated as belonging to an ATM channel CH.
In the case of AAL2, a channel having the same VPI/VCI address can contain up to 255 sub-channels, which are marked by so-called CIDs (Channel Identifiers). Sub-channels having different CIDs relate to the transmission of information for different subscribers. The Payload Type Identifier PTI for each ATM cell ATM4 in the case of AAL2 is “000”. With respect to FIG. 2, the ATM cell ATM4 first of all exhibits a header, which is followed by an OSF (Offset Field). The Offset Field indicates, where the first CID, in the present case CID1, is positioned. This is necessary because CIDs may be cell-overarching. CID1 is followed by field LI1, which indicates the length of the first payload “Payload CID1”. Next, there follows field UUI1 and field CRC1, which reflects the result of a cyclic redundancy check, i.e. a cyclic block test, across the header of CID1, which contains fields CID1, LI1, UUI1 and CRC1. After payload CID1 there follows the header of CID2 with fields CID2, LI2, UUI2, CRC2. This is followed by payload CID2, etc.
The encoding rule used for the transmission is determined in the UMTS network e.g. by the RNC node, depending on which requirements were addressed to it by a subscriber, e.g. by a mobile phone, particularly speech transmission or data transfer from the Internet. A monitoring apparatus, which serves monitoring purposes at the interface, therefore does not become aware of the agreed encoding rule without the finding, configuring and decoding the channels on which the control information (Control Plane Data) are transmitted.
In the prior art, measuring apparatuses, particularly for monitoring transmissions on ATM channels at an interface arranged between a first and a second node of a transmission system, therefore require manual configuration of the ATM channels. In this process, the user is asked to manually enter the VPI/VCI addresses and the associated AAL type of the channel he wants to monitor. In the case of AAL2, it is also necessary to configure all CIDs manually.
In order to check whether the address combination of the current ATM cell has already been recorded in the table, there are used in the prior art table search algorithms, which require an extremely fast and hence very expensive RAM memory and a high processor capacity, because the algorithm for the search itself is formulated in software.
This manual configuration exhibits two major disadvantages: Once made, a manual configuration is definite. An adaptation to changes in the configuration of the interface, which the user monitors, e.g. during the opening of new channels and the closing of existing channels, is not made. Also, the user does not know in most cases, which ATM channels exist at the interface to be monitored. By using the time-consuming manual configuration, which is also afflicted with errors, it is not possible to monitor channels at the interface whose VPI/VCI addresses are not known to the user.